The present invention generally relates to network interface units and communication systems, and more particularly to a network interface unit for connecting a token ring network to a communication network having a multi-conjunction architecture, and a communication network which uses such a network interface unit. Hence, the present invention is applicable to multi-media communication typified by a local area network (LAN).
A multi-channel lattice communication network which is applicable to LANs, public line networks and especially to a multi-media communication was previously proposed in U.S. Pat. No.4,839,887 in which the assignee is the same as the assignee of the present application. The proposed lattice communication network is analogous to nerve cells of a living body, and employs node apparatuses. Each node apparatus has a plurality of input and output channels and is used as a communication control element. Such node apparatuses are coupled in a multi-conjunction to form the lattice communication network. Each node apparatus transfers one of incoming digital signals according to a first-input-first-output logic.
The proposed lattice communication network has a large degree of freedom of network topology because of the multi-conjunction architecture. Thus, the survivability of data is high. In other words, even when a fault occurs in a path of the communication network, it is possible to carry out the communication through other paths. Furthermore, it is possible to select optimum paths for the communication because each node apparatus transfers the incoming digital signals according to the first-input-first-output logic.
On the other hand, the proposed lattice communication network employs a multi-channel system in which a plurality of connecting channels are established, so as to effectively achieve a full duplex communication. The proposed lattice communication network is effectively applicable to the physical layer to the network layer of the open system interconnection (OSI), for example.
As other networks, there is the token ring network employing the ring topology of IEEE 802.5. A description of the algorithm of this token ring network will be given later on in the specification. The token ring network has superior security functions in that all terminals are synchronized, only the terminal which has a free token (that is, a token which indicates the right to transmit) is authorized to make a transmission, message information is returned to a transmitting terminal thereby making it possible to confirm whether or not the message information is correctly received by a receiving terminal and the like. Moreover, the token ring network has a satisfactory throughput delay charactertistic when the traffic is high.
However, according to the token ring network, the transmitting terminal cannot start the transmission until the free token makes one round of the network, and the waiting time tends to become long. In addition, the performance of the token ring network deteriorates with system expansion. That is, in the token ring network, the time it takes for the free token to reach a terminal becomes longer as the number of terminals increases, and furthermore, the traffic becomes high at the same time, thereby making it even longer for the free token to reach a terminal. The token ring network also has a problem in that the survivability of data is poor because the network is greatly affected by a fault in a center ring.
On the other hand, the proposed lattice communication network does not have the concept of address, and can be regarded as "a cable which has a predetermined algorithm". In other words, the proposed lattice communication network does not require a fixed protocol, and it is thus possible to connect terminals which use various protocols. Accordingly, when a system is formed by connecting the terminals of the token ring network to the lattice communication network, it is possible to anticipate or expect that the problems of the token ring network will be eliminated and a token ring network which displays the advantageous features of the lattice communication network will be realized.
However, it is impossible to simply connect the terminals of the token ring network to the lattice communication network, as at least an interface unit is required to connect the token ring network and the lattice communication network. But as suce an interface unit has yet to be developed, there is a demand to realize such an interface unit.
In addition, a terminal of the token ring network generally outputs a phantom signal which produces a dc current on a lobe media or cable coupling the terminal to the token ring network, and a connecting condition between the terminal and the network via the cable is monitored according to the level of this dc current for a faulty condition, and after a normal connection is confirmed the terminal is connected to the network. Thus, it is necessary for a network interface unit, coupled to the token ring network, to output a level of such a current appropriate to the terminal, which current level must be within a predetermined range, so that a connecting condition between the terminal and the network is correctly monitored.
Moreover, prior to the output of a phantom signal, the terminal of the token ring network transmits a Lobe Media Test MAC frame (which is a kind of message information and hereinafter called a LMT frame) for detecting a condition of the cable (or lobe media) coupling the terminal and the network. If the cable is in a normal condition, the LMT frame transmitted by the terminal is immediately looped back from a concentrator of the token ring network. Thus, it is possible for the terminal to detect from the LMT frame which has been returned to the terminal that the lobe media is in a normal condition. Hence, in the case of the network interface unit for coupling the lattice communication network and the token ring network, there is a demand to provide means for looping a LMT frame back to the terminal when it has been received from the terminal, to achieve safe interface between the token ring network terminal and the lattice communication network.